In recent years, there is proposed a DC-DC converter having a magnetic-field canceling type transformer equipped with first and second windings that are wound around a core in reverse directions, and first and second switching elements connected to the windings (for example, see a patent reference 1). In the patent reference 1, currents flowing through the first and second windings are detected by a plurality of (two) current sensors to calculate a current difference therebetween, while the operation of the first and second switching elements are controlled so that the calculated current difference becomes zero.